1. The Field of the Invention
The present invention pertains to a method and system for verifying the functionality of venting or relief valves required to control internal pressure or external pressure (vacuum) in storage tanks to prevent the explosion or implosion thereof. More specifically, the invention is concerned with remotely and on-line checking of the set pressure of the relief valve by periodically attempting to open the valve and measuring the deviation from the set point of the force required to open the valve. It also provides for indicating unplanned events of valve venting.
2. The Prior Art
Bulk liquid industrial products with relatively low vapor pressure are normally stored in storage vessels and flat bottom tanks. These liquid containment vessels are designed for low-pressure services not to exceed 15 psig. Due to changes in the level within the storage vessel, upon filling and evacuation, of its liquid content and also to accommodate temperature variation, it becomes apparent that a vent nozzle is required to allow for vessel breathing. In nonhazardous service, such as water storage, a vent open to the atmosphere, such as a gooseneck, is an acceptable practice. For combustible commodities with minimum ecological adverse effects, an atmospheric vent with an in-line flame arrester will be considered as a minimum requirement for safe storage. For hazardous material, such as benzene (or other volatile petroleum products), an open vent is no longer safe nor environmentally acceptable. Therefore, vents from such storage vessels are connected to vapor recovery or destruction systems. Since these vapor mitigation means are subject to failure, vessels are protected with emergency relief valves to mitigate pressure upset conditions without damaging the vessel or causing the spillage of its contents. These relief valves should be designed for low pressure or low vacuum or combinations of low pressure and low vacuum. Some of these relief valves are weight-loaded while others are spring-loaded. In all such relief valves, a disk, commonly known as a pallet, engages a seat integral to the relief valve housing and mounted on a relief nozzle of the vessel. This pallet maintains a seal under a force imposed by a weight or a spring. The imposed force is at or below the allowable working pressure of the storage tank. The required pressure to initiate the lifting of the pallet and opening of the valve for venting is called "set pressure." These relief valves are passive devices that will open, or lift, in response to pressure changes within the vessel that exceed the set pressure. These valves allow for venting or inducing air (or inert gas) to maintain an acceptable pressure within the vessel.
The cost of a storage vessel is relatively proportional to its volume and its design pressure. For large vessels, it becomes economically prohibitive to design such vessels for more than few pounds of pressure. In fact, the internal design pressure of many of the large vessels is less than 3 psig with external design pressure (vacuum) of less than 1/2 psig (8 ounces). For a given tank shell thickness, the ability of the vessel cylinder to contain internal pressure is much greater than its ability to sustain external pressure. Therefore, relief valves for internal pressure are normally designed for a higher set point than that for external pressure.
Despite the fact that many vessels followed proper design guidelines and are provided with relief valves and other safety devices that should prevent their damage, it is not unusual to find imploded and ruptured storage vessels in tank farms and liquid loading and storage facilities. In fact, vessel failure in many of these cases is attributed to the failure of the relief valve to function. A relief valve malfunction is most often due material build up at the valve seat, causing the pallet to stick in place and not allowing it to lift properly for relief of the upset pressure condition.
This phenomenon has intrigued many inventors and inspired them to find means to monitor and test such relief valves to insure their proper function. Examples of known approaches to devise means to monitor relief valve action or to test its set pressure are described in the following patents.
U.S. Pat. No. 5,497,725 to Theisen et al describes a device for providing visual indication and record of the actuation of a tank vent opening. It comprises a rod contacting a tank vent pallet at one end, and a magnet affixed to the other end of the rod, which moves in a chamber adjacent to a vertically positioned rotationally mounted magnetic wafer with distinguishable sides. Movement of the magnet, occasioned by the opening tank vent, will flip the wafers, but will not affect the wafers when the magnet goes down after the vent is closed.
U.S. Pat. No. 4,255,967 to Grymonprez et al describes a local method for testing the setting of a valve, comprising a valve member biased toward a valve seat, a hydraulic or pneumatic pulling device is engaged with the valve member and operated to apply a tractive force opposed to the bias which is just sufficient to separate the valve member from the valve seat. This force is indicated by a suitable gauge and is proportional to the tractive force. The sum of the tractive force and the force acting on the valve from inside the vessel, as measured by another pressure gauge, is indicative of the setting of the valve.
U.S. Pat. No. 4,428,223 to Trevisan describes an apparatus for periodically testing the operation of safety valves on pressure apparatus or vessels and comprises a thrust generating assembly mounted on a load bearing structure for gripping the valve being tested. A dynamometer device measures the force applied by the thrust generator to the safety valve.
U.S. Pat. No. 4,761,999 to Thompson describes a test apparatus for measuring the set pressure of a valve coupled to a port in a pressure line. The valve includes a closure element movable between a first limit position sealing the port and a second limit position opening the port. A spring-loaded element is provided for biasing the closure element toward said first limit position. An expandable diaphragm device is provided which, upon the introduction of air pressure into an airtight chamber defined by a movable piston, the diaphragm and the base element, is adapted to apply a force to the closure element in a direction opposite to the direction of the bias provided by the spring load means. An element for calculating the set pressure of the valve is also provided.
U.S. Pat. No. 4,557,136 to Greenhalf; U.S. Pat. No. 5,620,024 to Yonezawa; U.S. Pat. No. 4,572,237 to Thompson; U.S. Pat. No. 4,651,670 to Silverwater; U.S. Pat. No. 5,275,036 to Schulz et al; and U.S. Pat. No. 4,949,288 to Bookout are similar inventions that comprise mechanical means for relief and safety valve testing and detection.